308 



THE DEVELOPMENT OF THE CHICK 



days, and its intermediate portion persists as an epithelial pocket 

 on the ventral face of the jugular vein (Fig. 178). This pocket 

 soon divides into dorsal and ventral moities of which the former 

 develops into the chief part of the thymus (thymus III) and the 

 latter into the so-called epithelial vestige III. (See below.) 



The fourth visceral pouch likewise separates from the pharynx 

 on the seventh day, and furnishes from its dorsal portion the 

 thymus IV, and from its ventral portion epithelial vestige IV. 

 (See below.) 



Fig. 178. — The derivatives of the embryonic pharynx of the chick. (After 

 Verdun from Maurer.) 



A. Of 7 days' incubation. 



B. Of 8 days' incubation. 



Ep. 3, Ep. 4, Epithelial bodies derived from the third and fourth visceral 

 pouches. J., Jugular vein, p'br (V)., Postbranchial bodies derived from 

 the fifth visceral pouch. Ph., Pharynx. Th. 3, Th. 4, Parts of the thymus 

 derived from the third and fourth visceral pouches respectively. T'r., Thy- 

 roid. HI, IV, third and fourth visceral clefts. 



The fifth pouch (postbranchial body) likewise becomes iso- 

 lated on the seventh day as a closed vesicle; its differentiation is 

 considered below. 



The Thymus. According to the above, the thymus of the 

 chick has a double origin on each side; the main portion (thymus 

 III) is derived from the dorsal wall of the intermediate part of 

 the third visceral pouch. This soon elongates to form an epi- 

 thelial cord extending along the jugular vein; a smaller portion 

 (thymus IV) of the thymus is derived from a corresponding part 

 of the fourth visceral pouch, and fuses with thymus III (Fig. 178). 



